Chemistry Reference
In-Depth Information
NH(a
1
Δ) + N
2
(X
1
Σg
+
)
HN
3
(X
1
A′)
H(
2
S) + N
3
(X
2
Πg)
N
LUMO+1 (a′)
N
N
H
N
N
N
LUMO (a′′)
H
1A′′
N
N
N
HOMO (a′)
H
N
N
N
HOMO-1 (a′′)
H
FIGURE 2.6.
Two different channels for HN
3
decomposition and the frontier orbitals
involved in the processes, calculated at the B3LYP/TZVP level of theory.
Why is this the case? High-level complete active space self-consistent field
(CASSCF) calculations yielded the shape of the excited-state potential energy
surfaces of HN
3
, which show that only the lowest singlet excited state has a small
barrier for nitrogen extrusion, while all of the other excited states and also the ground
state requires significant activation energy to generate the nitrene (imidogen).
111
Figure 2.6 shows the orbital energy diagram generated at the B3LYP/TZVP level of
theory for HN
3
. The highest-occupied molecular orbitals (HOMO) and lowest-
unoccupied molecular orbitals (LUMO) are orthogonal to each other due to their
different symmetry, and the lowest electronic band corresponds to excitation of an
electron from the HOMO to the LUMO, which is cited as the 1A
00
excited state. So,
this S
1
state is 1A
0
!
1A
00
, which is why the oscillator strength of this transition is
very small and in good agreement with the weak absorption band at 270 nm. Time-
dependent density functional theory (TD-B3LYP/TZVP) calculations predict this
transition to be about 260 nm, again in very good agreement with the experimental
observation.
Another very interesting feature was observed related to the rotational state
distribution of the photoproducts of HN
3
dissociation.
107
Imidogen shows very small
rotational excitation, whereas the nitrogen molecule has a very high number of
rotational states being populated. Why does one fragment remains rotationally
stable, while the other has high rotational motion upon generation? Thus far, there
is no answer to this question in the literature. An early theoretical report using
semiempirical (PM3) calculations
92
showed that the azide unit had a bent structure
with the proximal N1
N2 bond being increased significantly in the S
1
excited state,
when compared to the ground state. Later on, several calculations suggested that the
bent azide structure and the lengthening of the proximal N
N bond is an indicator of
nitrene formation.
60,74,90
Based on these theoretical calculations and the character-
istic geometric signature of the S
1
state, a possible explanation can be provided.
